In the oil and gas exploration and production industry long strings of jointed tubing or pipe are utilised to carry fluids between the surface and downhole locations within drilled bores, which strings and bores may be several kilometers long. In all downhole operations there is a small possibility of the pipe bore becoming restricted by, for example, cement residue or foreign objects such as a piece of wood or a metal bolt. In most cases this does not have any detrimental effect on operations. However, there are numerous tools and procedures that require a ball, dart or plug to travel through the pipe to perform a specific function downhole. Accordingly, prior to such operations it is necessary to inspect the pipe for the presence of any restrictions which would hold up the ball, dart or plug. Such inspections are normally achieved by checking the pipe string in stages as the string is pulled out of the bore and the pipe sections are separated at surface, before being reassembled in preparation for the operation involving the passage of the ball, dart or plug. Pipe strings are normally formed of large numbers of pipe sections that are typically around 10 meters long and have threaded ends. The pipe sections are often made up and stored as “stands”, each formed of three pipe sections, and thus around 30 meters long. Accordingly, when a pipe string is being pulled out of a bore, the string is lifted in 30 meter stages, to allow the uppermost stand to be removed.
One other commonly used method of checking the pipe bore for restrictions is to drop a hollow sleeve, of a slightly larger diameter than the ball, sleeve or plug, on a 40 m length of wire into the upper end of the pipe string. The pipe string is then pulled out of the bore to allow removal of the top pipe stand. If the wire is visible when the stand is separated from the string the operator knows that the sleeve is in the next stand and that the stand that has been separated from the string is unobstructed. This operation may be carried out relatively rapidly, but on many occasions the sleeve will not drop through the pipe, and the wire may become tangled or drop down such that it is not visible when the stand is separated. Thus, the drift and the obstruction point may go unnoticed.
In another method, an operator working at an elevated level simply drops an object, or drift, of a slightly larger diameter than the ball, sleeve or plug, through each pipe stand as it is being racked. The drift is retrieved at the bottom of the stand and then returned to the operator by means of the elevators used to lift the pipe out of the bore. This process is relatively slow, and it is not unknown for the drift to be dropped or otherwise fall, at significant risk to operators working below.
Bjørnstad U.S. Pat. No. 6,581,453 teaches a method of drifting pipe where the drift includes a radio transmitter or radioactive source. The drift is used in conjunction with a detection device positioned at surface to locate the position of the drift inside the drillpipe as the pipe is pulled from the hole. Such electronic detection of a drift has the drawback of being somewhat complicated, and the equipment would require to be physically robust. The equipment would also have to be intrinsically safe so as not to provide an ignition source. If the drift incorporated a radioactive source, regulations would require the drift to be handled and stored with great care. Bjørnstad also teaches a 30 m long drift in the form of a pipe that will be detected by default as the pipe is pulled from the hole. However, it is believed that the considerable weight of the drift and other issues would pose significant practical difficulties for an operator.
Polley U.S. Pat. No. 4,452,306 describes apparatus for detecting ruptures in drill pipe above and below the drill collar. The apparatus is deployed in response to surface loss in drilling pressure, indicative of washout in the drill pipe. The apparatus comprises a tool that may be pumped down through a drill pipe string to seat in a sub in the drill string above the drill collars. The drill pipe string is then pressurised above the tool to a predetermined pressure and the pressure held for a predetermined time. The pressure is monitored and, if the pressure holds, any rupture in the drill pipe is below the tool. If the pressure holds, the pressure in the string above the tool is increased to shear pins in the tool, allowing an actuator within the tool body to move and expose by-pass apertures. This allows fluid to drain from string as the string is retrieved to permit drill pipe repair below the drill collars. If, on the other hand, the drill pipe does not hold pressure above the tool, the drill pipe is pulled one section at a time. The stands are checked until the drill pipe washout is located. The damaged pipe is replaced and the drill string is tested again. If the pressure holds, the pressure is increased until the pins shear, to allow circulation through the tool. The tool may then be retrieved on wireline.
Morrill U.S. Pat. No. 5,343,946 describes a drop-in check valve used to re-establish control of a well in circumstances where there may be a gas build-up downhole. The valve is pumped from surface to lock into a landing sub provided in the string close to the bottom of the hole. The valve includes a ball that is pushed against a seat when the downhole pressure exceeds the pressure above the valve.
It is among the objectives of embodiments of the present invention to provide an efficient, technically simple and safe method for drifting tubing.